Is it a two stroke?have you tried spraying the bolt with releasing oil?I have found that brake fluid is a great releasing agent but be careful of your paintwork as it will strip it off to the metal.If it is too far gone to use some freeing oil on it , you may be able to get it out by heating up the aluminium around and underneath where the exhaust bolt passes through the casting. you will need to use a blow torch or a heat gun for this so, be careful.When I had to do something similar I had to empty the fuel tank and lay the scooter on its side.don't forget to clean the area of any oily deposits before applying any heat.I know that I have posted something similar on this topic before but I also know what a pain it is trying to find the right thread amongst so many others.My similar problems were with Vespa PX. In one case I had to saw /chisel the old exhaust bracket off and then applied heat around the casting , I then got the bolt to move a bit ,so I kept on tightening it back up and loosening it until it became free.Once it was out I ran a drill through the hole to enlarge it slightly and then before fitting the new exhaust bolt I packed the hole with copper grease.The worse one I have had to do (which I hope you don't have to) , the bolt was totally seized. I had to drill the bolt out, starting with a small drill to make a pilot hole and then going up a drill size at a time until I had completely drilled out the bolt. The upside was that due to all the drilling, it made the hole larger so once again , with a good blob of copper grease on the new bolt, it never seized again. Why the manufacturers cannot apply some copper grease to the bolt when the scooter is being assembled in the first place puzzles me.Let's hope that it is a simpler fix for you than the above. Good luck to you.Nigel.